The present invention relates to a method of manufacturing a phase shift mask, and more particularly to a method of manufacturing a spatial frequency modulation type phase shift mask. A mask for hyperfine patterning of a submicron unit has recently been required in accordance with high integration of all elements.
In order to meet this requirement, the phase shift mask has been developed, and the manufacturing theory on the phase shift mask is as follows.
As shown in FIG. 1, a phase shift film 1 is basically necessary for manufacturing the phase shift mask and the role of the phase shift film 1 is to shift the phase of light amplitude to be incident. Here, the number 2 indicates chrome Cr and the number 3 indicates silica Si.
FIG. 2 shows the light amplitude and a graph (a) indicates the light amplitude to be incident to silica 3 when the phase shift film 1 is not there, and a graph (b) indicates the light amplitude when the above film 1 is there.
In accordance with FIG. 2, the phase of the light amplitude is known to have been shifted 180.degree. by the phase shift film. Here, if n is the refractive index of the phase shift film, d the thickness, and n.sub.o the air refractive index, the phase difference .delta. of graph (a) and (b) (of FIG. 2) can be put into the following formula (1). ##EQU1##
At this time, the phase difference .delta. should be 180.degree., in order that the phase is perfectly shifted. If .pi. is substituted into the formula (1) instead of the phase difference .delta., the thickness of the phase shift film, to be perfectly shifted, can be put into the following formula (2) ##EQU2##
With reference to FIG. 3(a)-(d) and FIG. 4(a)-4(d), comparing the general mask with the phase shift mask, the state is as follows.
FIG. 3 (a) shows the state that the general masks 4 for pattern are aligned in parallel on the substrate, FIG. 3 (b) shows the light amplitude out of the mask 4, FIG. 3 (c) shows the light amplitude out of the substrate 5, and FIG. 3 (d) shows the light intensity.
As shown in FIG. 3 (b), the light amplitudes out of the mask 4 offset each other, and the classification of intensity is not clear on the substrate, because the difference of the light amplitude is slight, as shown in FIG. 3 (c) and FIG. 4 (d). Accordingly, when hyperfine patterning is executed, the degree of offset is larger, so that hyperfine patterning cannot be performed with the above mask 4.
FIG. 4 (a) shows the alignment state of the phase shift mask 7 having the phase shift film 6 between masks 4. FIG. 4 (b) shows the light amplitude from the mask 4, FIG. 4 (c) shows the light amplitude from the substrate 5, and FIG. 4 (d) shows the light intensity. As shown in FIG. 4 (c) and FIG. 4 (d), the difference of the light amplitude is bigger, so that the classification of intensity is clear, therefore hyperfine patterning is in a better position.
The different types of phase shift masks include a spatial frequency modulation type, an edge stress type and a cut-off effect stress type, and these will be explained with reference to FIG. 5 (a).about.FIG. 5 (f).
FIG. 5 (a) shows a spatial frequency modulation type. The patterning is performed after the chrome film 9 is formed on the silica substrate 8, and then the phase shift film 10 is formed between the patterned chrome film 9.
FIG. 5 (b) and FIG. 5(c) show the edge stress type. The phase shift film 10 is formed so as to shield the patterned chrome film 9, or the phase shift film 10 is formed on the chrome film 9.
FIG. 5 (d).about.FIG. 5 (f) show the cut-off effect stress type. The phase shift film 10 is formed between the patterned chrome film 9 and the chrome film 9, or the phase shift film is formed on the separated chrome film 19 after having been patterned and separated from the patterned chrome film 9 again, or the chrome film 9 is formed on the silica substrate 8 not to be etched after having been patterned in advance and repeatedly etched enough to the predetermined width.
With reference to FIG. 6 (a).about.FIG. 6 (d), the manufacturing process of a spatial frequency modulation type phase shift mask as a prior art of the present invention among the above 3 techniques will be explained.
FIG. 6 (a) is a plan view of the manufactured phase shift mask, and FIG. 6 (b).about.FIG. 6 (d) are cross-sectional views of manufacturing processes along the line a-a' of FIG. 6 (a), and FIG. 6 (e) is a cross-sectional view along the line b-b' of FIG. 6 (a).
First, the chrome film 9 is formed on the silica substrate 8, as in FIG. 6 (b), and then the process of photo/etch is performed on the chrome film 9 by using the negative photoresist 11 and the chrome film 9 is patterned. And, the phase shift film 10 is generally formed on the patterned chrome film 9, as in FIG. 6 (c), and then the phase shift film 10 is patterned by the dry-etch method, so that the above film 10 is left between the chrome film 9 and the adjacent chrome film 9, as in FIG. 6 (d). As a consequence, the manufacture of the phase shift mask is completed.
When the wafer spread with photoresist is developed by using the phase shift mask, as in FIG. 6 (a), the ideal pattern film 11, as in FIG. 7 (a), is not formed, but the pattern film, as in FIG. 7 (b), is formed. Namely, the normal pattern film 11 is formed in the direction of the line a-a' of FIG. 6 (a), but the unnecessary bridge pattern film 11a is formed in the edge part of the direction of the line b-b' having only the phase shift film 10 without the chrome film 9.
With reference to FIGS. 8 (a) and (b), the reason that the bridge pattern film 11a is to be formed, will be explained as follows.
As light is incident, the light amplitude has a predetermined positive level value in the part without the phase shift film 10, but the light amplitude has a negative level value, having the same absolute value as the above positive level value, on both sides of the edge part in the phase shift film 10. Accordingly, the light intensity to have been incident has a negative impulse value on both sides of the edge part in phase shift film 10, as in FIG. 8 (b). Therefore, as shown in FIG. 7 (b), the unnecessary bridge pattern film 11a is formed between the pattern films 11.
Further, control of the manufacturing process is difficult, owing to using the negative photoresist, when the predetermined wafer is patterned by using the above conventional spatial frequency modulation type phase shift mask.